Peculiarities of Relativistic Effects in 31P NMR Shielding Constants of Phosphines and Phosphine Chalcogenides.

Relativistic effects on 31P NMR shielding constants/chemical shifts in various phosphorus compounds can reach substantial magnitudes influencing the overall accuracy of 31P spectra modeling; therefore, it is of utmost importance to study these effects more closely. This paper reports on the dependences of relativistic corrections to 31P NMR shielding constants in a representative series of phosphines and phosphine chalcogenides with halogen and chalcogen substituents on the heaviness of the substituents on phosphorus (expressed by the total atomic number of atoms adjacent to phosphorus, TAN). The obtained dependences of relativistic corrections to phosphorus shielding constants on TAN for phosphines and phosphine chalcogenides were found to be cardinally different. This difference was explained on the basis of the third-order perturbational theory expression for one of the most important mechanisms of the relativistic heavy atom on light atom (HALA) effect, namely, the spin-orbit HALA (SO-HALA), formulated within the approximation of molecular orbitals. It was found that its main reason lies in the fact that the SO-HALA effect on 31P NMR shielding constants of phosphines is substantially suppressed as compared to that in phosphine chalcogenides due to the presence of a phosphorus lone electron pair.